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* TCP Vegas congestion control
* This is based on the congestion detection/avoidance scheme described in
* Lawrence S. Brakmo and Larry L. Peterson.
* "TCP Vegas: End to end congestion avoidance on a global internet."
* IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
* October 1995. Available from:
* See for their implementation.
* The main aspects that distinguish this implementation from the
* Arizona Vegas implementation are:
* o We do not change the loss detection or recovery mechanisms of
* Linux in any way. Linux already recovers from losses quite well,
* using fine-grained timers, NewReno, and FACK.
* o To avoid the performance penalty imposed by increasing cwnd
* only every-other RTT during slow start, we increase during
* every RTT during slow start, just like Reno.
* o Largely to allow continuous cwnd growth during slow start,
* we use the rate at which ACKs come back as the "actual"
* rate, rather than the rate at which data is sent.
* o To speed convergence to the right rate, we set the cwnd
* to achieve the right ("actual") rate when we exit slow start.
* o To filter out the noise caused by delayed ACKs, we use the
* minimum RTT sample observed during the last RTT to calculate
* the actual rate.
* o When the sender re-starts from idle, it waits until it has
* received ACKs for an entire flight of new data before making
* a cwnd adjustment decision. The original Vegas implementation
* assumed senders never went idle.
#include <linux/config.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/tcp_diag.h>
#include <net/tcp.h>
/* Default values of the Vegas variables, in fixed-point representation
* with V_PARAM_SHIFT bits to the right of the binary point.
#define V_PARAM_SHIFT 1
static int alpha = 1<<V_PARAM_SHIFT;
static int beta = 3<<V_PARAM_SHIFT;
static int gamma = 1<<V_PARAM_SHIFT;
module_param(alpha, int, 0644);
MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)");
module_param(beta, int, 0644);
MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)");
module_param(gamma, int, 0644);
MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");
/* Vegas variables */
struct vegas {
u32 beg_snd_nxt; /* right edge during last RTT */
u32 beg_snd_una; /* left edge during last RTT */
u32 beg_snd_cwnd; /* saves the size of the cwnd */
u8 doing_vegas_now;/* if true, do vegas for this RTT */
u16 cntRTT; /* # of RTTs measured within last RTT */
u32 minRTT; /* min of RTTs measured within last RTT (in usec) */
u32 baseRTT; /* the min of all Vegas RTT measurements seen (in usec) */
/* There are several situations when we must "re-start" Vegas:
* o when a connection is established
* o after an RTO
* o after fast recovery
* o when we send a packet and there is no outstanding
* unacknowledged data (restarting an idle connection)
* In these circumstances we cannot do a Vegas calculation at the
* end of the first RTT, because any calculation we do is using
* stale info -- both the saved cwnd and congestion feedback are
* stale.
* Instead we must wait until the completion of an RTT during
* which we actually receive ACKs.
static inline void vegas_enable(struct tcp_sock *tp)
struct vegas *vegas = tcp_ca(tp);
/* Begin taking Vegas samples next time we send something. */
vegas->doing_vegas_now = 1;
/* Set the beginning of the next send window. */
vegas->beg_snd_nxt = tp->snd_nxt;
vegas->cntRTT = 0;
vegas->minRTT = 0x7fffffff;
/* Stop taking Vegas samples for now. */
static inline void vegas_disable(struct tcp_sock *tp)
struct vegas *vegas = tcp_ca(tp);
vegas->doing_vegas_now = 0;
static void tcp_vegas_init(struct tcp_sock *tp)
struct vegas *vegas = tcp_ca(tp);
vegas->baseRTT = 0x7fffffff;
/* Do RTT sampling needed for Vegas.
* Basically we:
* o min-filter RTT samples from within an RTT to get the current
* propagation delay + queuing delay (we are min-filtering to try to
* avoid the effects of delayed ACKs)
* o min-filter RTT samples from a much longer window (forever for now)
* to find the propagation delay (baseRTT)
static void tcp_vegas_rtt_calc(struct tcp_sock *tp, u32 usrtt)
struct vegas *vegas = tcp_ca(tp);
u32 vrtt = usrtt + 1; /* Never allow zero rtt or baseRTT */
/* Filter to find propagation delay: */
if (vrtt < vegas->baseRTT)
vegas->baseRTT = vrtt;
/* Find the min RTT during the last RTT to find
* the current prop. delay + queuing delay:
vegas->minRTT = min(vegas->minRTT, vrtt);
static void tcp_vegas_state(struct tcp_sock *tp, u8 ca_state)
if (ca_state == TCP_CA_Open)
* If the connection is idle and we are restarting,
* then we don't want to do any Vegas calculations
* until we get fresh RTT samples. So when we
* restart, we reset our Vegas state to a clean
* slate. After we get acks for this flight of
* packets, _then_ we can make Vegas calculations
* again.
static void tcp_vegas_cwnd_event(struct tcp_sock *tp, enum tcp_ca_event event)
if (event == CA_EVENT_CWND_RESTART ||
static void tcp_vegas_cong_avoid(struct tcp_sock *tp, u32 ack,
u32 seq_rtt, u32 in_flight, int flag)
struct vegas *vegas = tcp_ca(tp);
if (!vegas->doing_vegas_now)
return tcp_reno_cong_avoid(tp, ack, seq_rtt, in_flight, flag);
/* The key players are v_beg_snd_una and v_beg_snd_nxt.
* These are so named because they represent the approximate values
* of snd_una and snd_nxt at the beginning of the current RTT. More
* precisely, they represent the amount of data sent during the RTT.
* At the end of the RTT, when we receive an ACK for v_beg_snd_nxt,
* we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding
* bytes of data have been ACKed during the course of the RTT, giving
* an "actual" rate of:
* (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration)
* Unfortunately, v_beg_snd_una is not exactly equal to snd_una,
* because delayed ACKs can cover more than one segment, so they
* don't line up nicely with the boundaries of RTTs.
* Another unfortunate fact of life is that delayed ACKs delay the
* advance of the left edge of our send window, so that the number
* of bytes we send in an RTT is often less than our cwnd will allow.
* So we keep track of our cwnd separately, in v_beg_snd_cwnd.
if (after(ack, vegas->beg_snd_nxt)) {
/* Do the Vegas once-per-RTT cwnd adjustment. */
u32 old_wnd, old_snd_cwnd;
/* Here old_wnd is essentially the window of data that was
* sent during the previous RTT, and has all
* been acknowledged in the course of the RTT that ended
* with the ACK we just received. Likewise, old_snd_cwnd
* is the cwnd during the previous RTT.
old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) /
old_snd_cwnd = vegas->beg_snd_cwnd;
/* Save the extent of the current window so we can use this
* at the end of the next RTT.
vegas->beg_snd_una = vegas->beg_snd_nxt;
vegas->beg_snd_nxt = tp->snd_nxt;
vegas->beg_snd_cwnd = tp->snd_cwnd;
/* Take into account the current RTT sample too, to
* decrease the impact of delayed acks. This double counts
* this sample since we count it for the next window as well,
* but that's not too awful, since we're taking the min,
* rather than averaging.
tcp_vegas_rtt_calc(tp, seq_rtt*1000);
/* We do the Vegas calculations only if we got enough RTT
* samples that we can be reasonably sure that we got
* at least one RTT sample that wasn't from a delayed ACK.
* If we only had 2 samples total,
* then that means we're getting only 1 ACK per RTT, which
* means they're almost certainly delayed ACKs.
* If we have 3 samples, we should be OK.
if (vegas->cntRTT <= 2) {
/* We don't have enough RTT samples to do the Vegas
* calculation, so we'll behave like Reno.
if (tp->snd_cwnd > tp->snd_ssthresh)
} else {
u32 rtt, target_cwnd, diff;
/* We have enough RTT samples, so, using the Vegas
* algorithm, we determine if we should increase or
* decrease cwnd, and by how much.
/* Pluck out the RTT we are using for the Vegas
* calculations. This is the min RTT seen during the
* last RTT. Taking the min filters out the effects
* of delayed ACKs, at the cost of noticing congestion
* a bit later.
rtt = vegas->minRTT;
/* Calculate the cwnd we should have, if we weren't
* going too fast.
* This is:
* (actual rate in segments) * baseRTT
* We keep it as a fixed point number with
* V_PARAM_SHIFT bits to the right of the binary point.
target_cwnd = ((old_wnd * vegas->baseRTT)
<< V_PARAM_SHIFT) / rtt;
/* Calculate the difference between the window we had,
* and the window we would like to have. This quantity
* is the "Diff" from the Arizona Vegas papers.
* Again, this is a fixed point number with
* V_PARAM_SHIFT bits to the right of the binary
* point.
diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd;
if (tp->snd_cwnd < tp->snd_ssthresh) {
/* Slow start. */
if (diff > gamma) {
/* Going too fast. Time to slow down
* and switch to congestion avoidance.
tp->snd_ssthresh = 2;
/* Set cwnd to match the actual rate
* exactly:
* cwnd = (actual rate) * baseRTT
* Then we add 1 because the integer
* truncation robs us of full link
* utilization.
tp->snd_cwnd = min(tp->snd_cwnd,
(target_cwnd >>
} else {
/* Congestion avoidance. */
u32 next_snd_cwnd;
/* Figure out where we would like cwnd
* to be.
if (diff > beta) {
/* The old window was too fast, so
* we slow down.
next_snd_cwnd = old_snd_cwnd - 1;
} else if (diff < alpha) {
/* We don't have enough extra packets
* in the network, so speed up.
next_snd_cwnd = old_snd_cwnd + 1;
} else {
/* Sending just as fast as we
* should be.
next_snd_cwnd = old_snd_cwnd;
/* Adjust cwnd upward or downward, toward the
* desired value.
if (next_snd_cwnd > tp->snd_cwnd)
else if (next_snd_cwnd < tp->snd_cwnd)
/* Wipe the slate clean for the next RTT. */
vegas->cntRTT = 0;
vegas->minRTT = 0x7fffffff;
/* The following code is executed for every ack we receive,
* except for conditions checked in should_advance_cwnd()
* before the call to tcp_cong_avoid(). Mainly this means that
* we only execute this code if the ack actually acked some
* data.
/* If we are in slow start, increase our cwnd in response to this ACK.
* (If we are not in slow start then we are in congestion avoidance,
* and adjust our congestion window only once per RTT. See the code
* above.)
if (tp->snd_cwnd <= tp->snd_ssthresh)
/* to keep cwnd from growing without bound */
tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp);
/* Make sure that we are never so timid as to reduce our cwnd below
* 2 MSS.
* Going below 2 MSS would risk huge delayed ACKs from our receiver.
tp->snd_cwnd = max(tp->snd_cwnd, 2U);
/* Extract info for Tcp socket info provided via netlink. */
static void tcp_vegas_get_info(struct tcp_sock *tp, u32 ext,
struct sk_buff *skb)
const struct vegas *ca = tcp_ca(tp);
if (ext & (1<<(TCPDIAG_VEGASINFO-1))) {
struct tcpvegas_info *info;
info->tcpv_enabled = ca->doing_vegas_now;
info->tcpv_rttcnt = ca->cntRTT;
info->tcpv_rtt = ca->baseRTT;
info->tcpv_minrtt = ca->minRTT;
rtattr_failure: ;
static struct tcp_congestion_ops tcp_vegas = {
.init = tcp_vegas_init,
.ssthresh = tcp_reno_ssthresh,
.cong_avoid = tcp_vegas_cong_avoid,
.min_cwnd = tcp_reno_min_cwnd,
.rtt_sample = tcp_vegas_rtt_calc,
.set_state = tcp_vegas_state,
.cwnd_event = tcp_vegas_cwnd_event,
.get_info = tcp_vegas_get_info,
.owner = THIS_MODULE,
.name = "vegas",
static int __init tcp_vegas_register(void)
BUG_ON(sizeof(struct vegas) > TCP_CA_PRIV_SIZE);
return 0;
static void __exit tcp_vegas_unregister(void)
MODULE_AUTHOR("Stephen Hemminger");